1. Field of the Invention
The present invention relates to the field of well logging. More specifically, the present invention relates to methods for determining wettability of oil reservoir using nuclear magnetic resonance data.
2. Background Art
Wettability is the tendency of a fluid to spread on and preferentially adhere to or xe2x80x9cwetxe2x80x9d a solid surface in the presence of other immiscible fluids. Knowledge of the wettability of an oil reservoir is important to reservoir and production engineers because reservoir wettability influences reservoir properties such as residual oil saturation, relative permeability, and capillary pressure. See F. G. Craig in the Society of Professional Engineers (SPE) Monograph on xe2x80x9cThe Reservoir Engineering Aspects of Waterfloodingxe2x80x9d, 1971. Thus, reservoir wettability information is crucial for efficient oil recovery. This information is becoming increasingly important as secondary and tertiary recovery methods are used to recover remaining reserves in old producing fields. For example, in a water-wet reservoir, a waterflood can be an efficient method of recovering remaining reserves after primary production, whereas, in a mixed-wet reservoir, a surfactant flood would be more effective in recovering the remaining oil.
Two widely-used wettability indicators are contact angles in water-oil-solid systems and the Amott wettability index. In addition, other laboratory tests for wettability are also available, including imbibition measurements. However, these are laboratory measurements and cannot be performed downhole. In contrast, a nuclear magnetic resonance (NMR) approach can provide a qualitative wettability indicator and has the advantage of being able to assess the fluids and rock at reservoir conditions.
NMR measurements on fluid saturated rocks are sensitive to the wettability of the rock matrix because relaxation rates of fluid molecules are enhanced when they are in contact with rock surfaces. This is because rock surfaces often have paramagnetic ions or magnetic ions which can provide efficient relaxation for the fluid molecules. Reservoir wettability not only depends on the inherent property of the rock matrix, but also on surface interactions between the rock matrix and the fluid molecules, i.e., it also depends on the attractive Coulomb forces that exist between the polar oil molecules and those on rock matrix surfaces.
Many laboratory NMR wettability studies have been reported in the literature. The first NMR study on wettability was by Brown and Fatt, who made T1 relaxation measurements on water-saturated unconsolidated sand packs constructed with different fractions of water-wet and oil-wet sand grains. See R. J. S. Brown and I. Fatt, xe2x80x9cMeasurements of Fractional Wettability of Oilfield Rocks by the Nuclear Magnetic Relaxation Method,xe2x80x9d Petroleum Transactions, AIME, 207, pp. 262-264, 1956. Numerous studies on the application of NMR to wettability have been published since then. See Q. Zhang, C. C. Huang, and G. J. Hirasaki, xe2x80x9cInterpretation of Wettability in Sandstones with NMR Analysis,xe2x80x9d Petrophysics, May-June, 2000, vol. 41, No. 3, pp. 223-233.
Prior NMR studies of wettability of partially saturated reservoir rocks have been mostly limited to rocks saturated with brine and low viscosity hydrocarbons, such as Soltrol, decane, and dodecane. These low viscosity fluids ae characterized as having narrow T1 and T2 distributions and long relaxation times. Accordingly, it is relatively simple to distinguish the hydrocarbon signal from the brine signal in the relaxation time distributions of partially saturated rocks. The oil relaxation times in the rocks can then be compared with those of the bulk hydrocarbon (i.e., outside the rock) to infer whether the oil is wetting the surface. However, wettability inferred from experiments using refined or pure hydrocarbons is not indicative of the wettability of the same rocks saturated with crude oil, because crude oils may contain asphaltenes and resins, which are known to have surface-active polar molecules that are attracted to opposite charge sites on the pore surfaces.
The above described approaches to the determination of rock wettability use laboratory measurements. Reservoir wettability determination from laboratory measurements is not definitive because it is not possible to accurately mimic reservoir conditions in the laboratory. In fact, the very processes required to obtain laboratory samples can alter the reservoir wettability. See N. R. Morrow, xe2x80x9cWettability and Its Effect on Oil Recoveryxe2x80x9d in the J. of Pet. Tech., December, 1990, pp. 1476 1484. Therefore, it is desirable to have methods that can determine reservoir wettability under downhole conditions.
One aspect of the invention relates to methods for determining reservoir wettability under downhole conditions. A method for determining downhole reservoir wettability according to the invention includes acquiring a first set of NMR measurements of formation fluids in earth formations at a selected axial depth; inverting the first set of the NMR measurements to produce a first distribution of a spin relaxation parameter for a fluid component in the formation fluids; acquiring a second set of NMR measurements of a formation fluid sample removed by a formation fluid testing tool at the selected axial depth, the formation fluid sample being kept at a substantially same pressure and temperature as those of the formation fluids in the earth formations at the selected axial depth; inverting the second set of NMR measurements to produce a second distribution of the spin relaxation parameter for the fluid component in the formation fluid sample; determining the reservoir wettability from a comparison of the first and second distributions of the spin relaxation parameter.
Other aspects of the invention would become apparent from the following description, the drawings, and the claims.